1. Field
Embodiments of the present invention relate to a method and apparatus for co-curing composite stringers.
2. Related Art
Aircraft stringers are elongated, thin strips of wood, metal, or carbon fiber to which a skin of an aircraft is fastened. For example, stringers can be attached within nacelles, fuselages, or wings of an aircraft to provide strength and support to the structure. Until recently, most stringers have been formed of metal. Now many stringers are formed of composite materials to reduce the aircrafts weight.
Some composite stringers are hollow and have a substantially trapezoid-shaped cross-section, which may be fixed or cured to a skin laminate. Furthermore, the skin laminate may serve as one of the stringer's walls. The skin laminate wall is typically wider and longer than the rest of the stringer walls, forming a plurality of flanges extending in a plurality of directions outward from the stringer. These flanges are used to attach the stringer to adjoining fuselage structure.
One method of producing a composite stringer includes the steps of placing uncured stringer charges or plies of composite material in a trough of a lay-up tool. The trough may include a base surface and two side surfaces extending upward therefrom at an angle above zero and below 90 degrees. Then a flexible hollow bladder is placed into the trough, over the composite material, and an uncured skin laminate is placed atop the bladder, mating with peripheral portions of the composite material. A vacuum bag is then placed over the skin laminate and sealed to the lay-up tool. Vacuum and heat via an autoclave can then be used to co-cure the composite material and the skin laminate into a monolithic structure. Pressure is provided against an internal surface of the stringer by the bladder and to an external surface of the skin laminate by the vacuum bag.
During the cure cycle, the bladder must be vented to the autoclave so that pressure is applied to the inside of the stringers. The bladders are generally vented out one or both opposing ends of the stringer. The flanges formed by the skin laminate proximate each of the opposing ends of the stringer must be smooth and precisely shaped. Because the bladder is being vented in the vicinity of these flanges, maintaining the smoothness and pressure desired is difficult.
Accordingly, there is a need for an apparatus and method for forming a co-cured stringer that overcomes the limitations of the prior art.